What are the future propulsion systems for interplanetary travel?

In a few decades, enhanced versions of current propulsion technology could reduce travel time to Mars from about a year to a few months…

By Leda Zimmerman

The current methods for space travel haven’t changed much in the four decades since we landed on the moon, says Paulo Lozano, H.N. Slater Assistant Professor of Aeronautics and Astronautics—though they continue to work well enough to send satellites into space, and take humans 300-400 kilometers above Earth in relative safety.

Current spaceflight depends on a rocket that burns fuel and oxidizer, which turns out to be both expensive and deficient as a means of propulsion for long-distance space travel, explains Lozano. Chemical-based rockets get terrible fuel efficiency, achieving very little thrust per kilogram of propellant used, and their exhaust velocity can’t exceed 5,000 meters per second. Using these tools, Lozano adds, it would take at least nine months to get to Mars (if your timing and the planets’ alignment are just right), and “the rockets would be huge compared to the payload.”

An alternative is on the horizon, though: the plasma rocket. “Instead of burning fuel, we ionize it, ripping electrons from atoms in the propellant,” says Lozano. The rockets use gases like xenon or krypton—the ones on the right side of the periodic table—and an electrical source accelerates the ions in the gas to create plasma. In this scheme, the higher the voltage exciting the plasma, the more velocity a rocket can achieve.

NASA has begun using a version of this kind of propulsion system for non-human space exploration, with solar arrays providing a limited but steady source of electricity for space missions that last years. But future generations of ion engines could deliver the goods for the kind of space voyages humans have long imagined, says Lozano. “There’s no impediment to applying thousands of volts to charged particles, and instead of moving five thousand meters per second, we can now have an exhaust moving at several tens of thousands meters per second, or more.” Compact and efficient nuclear reactors on board could provide the electric juice for ion engines propelling cargos swiftly from point to point in our solar system.

We’d still need the power of a chemical rocket to break the bonds of Earth, though. Ultimately, we might “take a chemical rocket taxi to low earth orbit,” says Lozano, and then “get on the high speed train, the rocket with the ion engine” to other planets.

At MIT’s Space Propulsion Lab, Lozano is working on small-scale, super-efficient thrusters for satellites. He credits the movies in part for his fascination with space travel, and specifically, with propulsion: “I saw in Star Wars that the rocket was the most important part. To escape the bad guys or explore new worlds, you needed rockets.” Personally, Lozano leans toward a combination of robotic and human discovery missions, and looks forward to a time when new propulsion systems “bring huge robotic space craft to the moons of Jupiter and Saturn, and explore these fascinating and quite exotic worlds.”